1. Field of the Invention
The present invention relates to an output circuit for a bus. More particularly, the present invention relates to an output circuit for a bus intended for consumer electronics control (CEC), which connects the control system of the entire system in a high definition multimedia interface (HDMI) used in, for example, digital input/output of a digital television.
2. Description of Related Art
The HDMI is used for digital input and output of a digital television. The HDMI connects a DVD player, a high-vision tuner, an AV amplifier and the like to a digital television. A single HDMI cable enables transmission of images, sounds and control signals (for example, refer to Japanese Patent Application Publication No. 2006-33436).
A control system for equipment connected by the HDMI is connected through a CEC bus. The CEC bus is a bi-directional bus.
FIG. 10 is a diagram showing an example of the configuration of an output circuit for a bi-directional bus for a CEC bus. The output circuit for a CEC bus 7A of related art has a resistor 1C, a Schottky diode 2A, an npn transistor 41, a resistor 42, and an output control circuit 5. An output node 6 is connected to the CEC bus 7A.
The Schottky diode 2A is provided for preventing a current from flowing back from the CEC bus 7A to a power source when power is off, and connected in series to the resistor 1C. One end of this series circuit is connected to a power supply potential Vcc and the other end is connected to the output node 6. In the specification of the CEC bus 7A, the current allowed to flow while power is off is defined as 1.8 μA, and the internal pull-up resistance is defined as 27KΩ±5%. The resistor 1C and the Schottky diode 2A are required to satisfy this specification.
Between the output node 6 and a grounding potential Gnd, the npn transistor 41 and the resistor 42 are connected in series, and an output control signal is inputted to the base of the npn transistor 41. When the output control signal outputted from the output control circuit 5 becomes a high level, the electrical continuity between the emitter and the collector of the npn transistor 41 is completed, and a low level is outputted from the output node 6 to the CEC bus 7A. When the output control signal becomes low level, the npn transistor 41 is turned off, and a high level is outputted from the output node 6.
The Schottky diode 2A operates to prevent interference of the CEC functions of other equipment connected to the output node 6 while power is off.